1. Field of the Invention
The present invention relates to an X-ray optical apparatus, and more particularly, to an optical apparatus in which a part of an X-ray optical system and a part of a visible optical system are provided with the same optical axis.
2. Related Background Art
There is known an X-ray microscopic apparatus employing approximately 2 to 5 nm of wavelength and which can observe a living biological sample and the like enclosed in a sample capsule under higher power in comparison with an optical microscope.
Soft X-rays are those of which have normally not less than 200 pm of wavelength among X-rays having 1 pm to 10 nm of wavelength. Such soft X-rays are electromagnetic waves which are roughly two orders of magnitude shorter than a visible radiation having 400 to 800 nm of wavelength. Such X-rays can transmit through a substance surface without being reflected thereby. However, they can be well absorbed by the substance and have a property of being rapidly decayed even in the air. Accordingly, a transmission observation by the use of soft X-rays is generally conducted in an X-ray apparatus employing soft X-rays, wherein the optical path of the soft X-rays is maintained in a high degree of vacuum. The optical element employs Fresnel's zone pattern and extremely thin samples are generally used. FIG. 4 is a view illustrating an example of a conventional X-ray microscopic optical system equipped with an X-ray optical system and a visible optical system. In this optical microscope, a sample set in the microscope can be observed by the use of both optical systems, so that low power preliminary observation by a visible radiation or a combination of high resolution observation by soft X-rays and a color information observation by visible radiation can be conducted before observation under high power is conducted by soft X-rays.
In FIG. 4, an X-ray optical system 11, an observation sample S held in holder H and a part of visible optical system 12 are placed in a vacuum vessel 13. A part of an optical axis of the visible optical system 12 is constituted to coincide with an optical axis of the X-ray optical system which passes through the observation sample. The X-ray optical system 11 includes a soft X-ray generator G, a condenser zone plate R11, an objective zone plate R12 and a camera element C. The soft X-rays from the soft X-ray generator G can be converged on the observation sample S with condenser zone plate R11. An image of the observation sample S irradiated with the soft X-rays is formed on the image element C by the objective zone plate R12. The objective zone plate R12 is structurally similar to the condenser zone plate R11 and is shown in FIG. 5. This objective zone plate R12 is attached releasably on the optical axis and comprises a base plate W having a generally square aperture P at the center thereof for passing the soft X-rays, a thin silicon nitride membrane N covering the aperture P of the base plate W and the Fresnel's zone pattern (not shown) formed on this silicon nitride membrane N.
The visible optical system 12 includes a lamp B, a first condenser L11, a first mirror M11 and a second condenser lens L12 each having an aperture at the center thereof for passing the soft X-rays, an objective lens L13 and a second mirror M12 each having an aperture at the center thereof for passing the soft X-rays, a reticule r10 and an eyepiece L14. A luminous flux for illumination from a lamp B will be projected on the observation sample S through the first condenser lens L11, the first mirror M11 and the second condenser lens L12. The image of the observation sample S is provided by the luminous flux for illumination. This image is formed on the reticule r10 through the objective lens L13 and the second mirror M12 and can be observed through the eyepiece L14.
Conventional known objective zone plate R12, as shown in FIG. 5, can scarcely transmit the visible radiation. In the X-ray microscope in FIG. 4, the objective zone plate R12 is, therefore, attached to a slide frame for taking it into and from a visible radiation path, and which is retracted from the visible radiation path during observation by the visible optical system 12. Accordingly, such arrangement has the disadvantage of complex structure. In addition, in the X-ray optical system 11, there is provided a long light path with a fine soft X-ray beam. Positioning the objective zone plate R12 and angle adjusting thus require a relatively high precision. This, coupled with the fact that the mechanism of the objective zone plate must effect retracting and resetting of the zone plate R12 in a highly evacuated vessel 13, substantially increases the cost of the X-ray optical apparatus.